1. Field of Invention
This invention relates to a method and apparatus for noninvasively measuring blood constituents and, more particularly, to a method and apparatus for detecting variations in the detected amplitude of one or more wavelengths of light which are transmitted through body tissue to measure the concentration of blood constituents such as saturated hemoglobin oxygen and for compensating for ambient light interference which adversely affects such determinations.
2. Description of the Prior Art
The determination of blood oxygen concentrations and the concentrations of other blood constituents such as injected dyes has become increasingly interesting to physicians and of increasing importance in the practice of clinical medicine. Generally, it is known to use spectrophotometric techniques to measure arterial hemoglobin oxygen saturation. For example, various blood constituent measuring devices and methods using non-invasive techniques are known whereby emitted light is passed through the sample, or reflected therefrom, and then detected by light sensors. Variations in the detected light at various wavelengths are then used to determine arterial oxygen saturation and/or pulse rates. Such devices and/or methods are shown, for example, in U.S. Pat. Nos. 4,407,290; 4,266,554; 4,167,331; 4,086,915; 3,998,550; and 3,647,299; and European patent Nos. EP 0 104 771 A3 and EP 0 102 816 A3.
However, such prior art oximetry devices and methods are typically inaccurate, for significant errors are induced in clinical oximetry if the classical absorption equation (Bier's Law) is used to calculate the saturation of oxygen as applicable to pure hemoglobin. The methods disclosed in U.S. Pat. Nos. 4,167,331; 3,998,550; 4,086,915; and 4,266,554 do not compensate such errors. As a partial solution to this problem, however, other patents teach the use of a mathematical approximation to Bier's Law by using ratios of the pulsating absorbance and the non-pulsating absorbance components of each of several wavelengths of transmitted light, as in U.S. Pat. Nos. 4,407,290 and 3,647,299 and European Patent Nos. EP 0 104 771 A3 and EP 0 102 816 A3, for example. Also, because the light absorption of tissue does not exactly correspond to that predicted by Bier's Law, some type of empirical calibration has been performed as taught in U.S. Pat. Nos. 4,407,290; 4,167,331; and 4,086,915.
The derivation of the absorbance in the pulsating component may be performed in different ways. One technique relies upon the quantitative measurement in the change in absorbance at each wavelength, as in U.S. Pat. No. 4,407,290 and European patent Nos. EP 0 104 771 A3 and Ep 0 102 816 A3. It is also known that the derivative of the change in absorbance and a peak to peak measurement of the pulsating absorbance component may be used to calculate the Oxygen content of arterial blood, as taught in U.S. Pat. Nos. 4,407,290 and 4,167,331.
In addition, it is known that a single light detector may be used. However, when a single light detector is used, the detected light for each wavelength must be separated. This is accomplished by using time separation and synchronous detection as taught in U.S. Pat. Nos. 4,407,290; 4,266,554; and 3,647,299, for example. However, because the light detectors also detect ambient light, some type of ambient light rejection technique is normally employed. One technique is to use four clock states and to allow for the subtraction of ambient light, as taught in U.S. Pat. Nos. 4,407,290 and 4,266,554. Another technique is to remove the non-pulsating absorbance component since ambient light is usually a non-pulsating absorbance frequency, as taught in U.S. Pat. Nos. 4,167,331 and 3,998,550. These techniques consider the ambient light to have a constant amplitude.
However, the techniques and devices disclosed in the above-referenced patents are not completely satisfactory and are deficient in several areas for the following reasons. Namely, Bier's Law and/or the use of empirical estimates usually only approximate the oxygen content of blood in living tissues in clinical environments. Moreover, prior art techniques for removing ambient light and motion artifacts are unsatisfactory and generally produce decreased signal to noise ratios and increased errors in the clinical measurements because up to fifty percent of the duty cycle is devoted to making ambient light measurements. In addition, techniques using synchronous detectors are not completely satisfactory since they require wide bandwidth AC amplifiers and because they also may devote a significant portion of the duty cycle, as much as 50%, to measuring the ambient light rather than the desired absorbance changes. Furthermore, the wide bandwidth requirements of the prior art devices render such devices more susceptible to frequency interference such as BOVIE interference, which is a type of system noise produced by electrical surgical devices such as coagulators and cauterizers. It typically affects the frequency range of 0.5 to 5 MHZ, but it can be found even in direct current. Although there is less energy in frequencies between DC and 0.5 MHZ, there is still enough energy to potentially cause interference which impairs the performance of many prior art devices. Previous embodiments also frequently require an analog channel for each wavelength and require that these channels be matched over the bandwidth. The resulting analog requirements may be so stringent as to require that both channels have a "normalized" DC output, as in U.S. Pat. No. 4,407,290.
Thus, although blood constituent measuring devices and methods have heretofore been suggested and have achieved some success, a need still exists for a device and method for more accurately measuring blood constituents such as oxygen saturation of blood in patients, particularly anemic patients and those with blood conditions such as low red blood cell counts. These blood counts are different from the assumed counts upon Which the factory "presets" are based, and accordingly, correcting the blood counts to account for patient anemia in the saturation equations, for example, could be a critical factor in keeping an anemic's brain and heart alive during surgery. Moreover, there is also a need for a device which is less sensitive to other types of interference, such as BOVIE interference, which can virtually interrupt oximetry at critical times during an operation. The present invention has been designed to meet these needs.